The present invention is directed to a simple system for sequentially allocating the use of a data bus to individual members of a series of data communication devices. There are presently available various communication devices such as computer terminals, printers and the like which are capable of transmitting and/or receiving data over a data bus. In addition, there are presently available various communications devices for remotely reading such instruments as electrical meters, gas meters, and the like. One such device is illustrated and described in U.S. Pat. No. 4,007,454 to Cain et al. Generally, and as use herein, these devices can be activated or initiated by the receipt of a trigger voltage and can be made to stop drawing current when the data exchange is completed. There are basically two known methods for sequentially acquiring data from a plurality of such data communication devices.
In one, each communication device is equipped with a data bank where data is continuously accumulated and stored in some type of storage member. When the storage member is addressed by a computer, it feeds back the information stored therein. Sequencing of a plurality of such devices is thus accomplished by the computer itself. An example of this type of system is shown in U.S. Pat. No. 3,962,545 to Abe and in U.S. Pat. No. 4,059,727 to Kingswell et al. In these systems the data storage bank and the computer addressing make the systems complicated and expensive, even though the resulting signals may be transmitted over a common telephone line.
In a second approach, a central computer or addressing device is connected to each data communication device by separate wire leads. Then the central computer or addressing device is pre-set to address each communication device according to a preferred pattern. This requires an excessive amount of wire and the installation thereof, is very difficult to add instruments once initial wiring is completed, and is relatively slow compared to the present system.
One further system is illustrated and described in the Neuville et al U.S. Pat. No. 3,662,366 in which a polling system includes a plurality of relatively complex transfer members. In this particular system the electrical system includes each device to be read which has alternating conducting and non-conducting areas thereon. Each transfer member includes a pair of contacts in engagement with the surface of the device to be read. As each transfer member is successively activated in a series of successive scanning cycles, a computer is able to tell the condition (on or off) of each device being scanned. No data is put onto a data bus, bidirectional flow of information cannot take place and each transfer member is activated for a prescribed time interval as preset in the transfer member.
In the present invention, on the other hand, a relatively simple approach is utilized for sequentially allocating the use of a data bus to individual members of a series of data communication devices. The bus used may be parallel (as for communication between microprocessers), or serial (as for example RS-232C as widely used in telephonic data communication) and the data may be analog or serial and may be as complex as desired. In the present invention, as differentiated from the Neuville approach, the receipt of a control signal causes a transfer station to tap a voltage from the supply line, to gate this voltage to its corresponding data communication device so long as current is drawn from the supply line by the data device, and to permit the data device to transmit data onto the bus during the time interval (which may be of arbitrary and/or differing duration) as determined by the device being controlled. Upon completion, the data communication device then ceases to draw current, causing the transfer member to pass the control voltage onto the next-in-line station. Each device in the series may receive data from ("listen to") the bus at all times during this process; when (and only when) its turn is reached the device may transmit data onto ("talk to") the bus as long as it continues to draw current from the supply line.
In general then the present system is directed to the allocation of data transmission opportunity among members of a plurality of data communication devices in accordance with a prescribed sequence. Rather than having the sequencing controlled by a central addressing system, each station on the electrical line activates the next station at the completion of transmission with no appreciable delays. Toward this end there is provided a voltage source and a main electrical cable including at least a supply line and preferably at least one separate data line. The supply line connects the voltage source to a plurality of control means, each of which is electrically connected to a corresponding data communication device by an electrical branch cable having at least a supply line and preferably one data line. Each of the control means includes a switch device operable between a normal position and a second position for tapping current from the main cable supply line and gating the current at the same voltage to the supply line of the corresponding branch cable and thus on to the data communication device, thereby activating the same to transmit data onto the branch data line and back onto the main data line or bus. Alternatively, as will be explained below, the gating of current to a branch line may be so arranged that the supply line current is temporarily withheld from the downstream stations. Once the upstream or "activated" data communication device has completed transmission of its data, a silicon controlled rectifier or some other switching device either already available or built into the data communication device changes its state, thus terminating the flow of current therethrough. A current sensing means detects the lack of current through the branch cable, returns the switching means to the normal (non-conducting) position, and signals the next-in-line control means to be activated. Thus the present invention allows the inexpensive construction of a simple multi-port data link. Note that in the foregoing, "voltage" and "current" have been referred to. It will be clear to those skilled in the art that the two terms are interchangeable in this discussion because it is the term "power" that is really the significant term, rather than either of its components. The three terms may be referred to hereinafter, but are considered interchangeable.
One way, of course, to trigger the entire sequence is to change the voltage on the supply line from zero to a designated high level which will initiate a sequence of readings from the entire line of data communication devices. Alternately the supply line can substantially be held at a maximum voltage level and the sequence can be triggered when desired with no more than a brief negative pulse on the supply line, whereby the supply line may be used to power other devices when the sequencing is not in progress. As another alternative, individual control means may be arranged to respond only to supply voltages above a predetermined threshold, below which voltage level the supply line may be used for other purposes.
The data communication devices are herein assumed to be generally low power devices which can derive all of their operating power from the supply line. This is not a strict requirement, however, as other types of data communication devices might simply sink current or switch current to ground to indicate that they are, or are about to be, putting out data, while deriving their operations power from other sources.
When the system is not being used to transmit data as described above, the same lines are available for other purposes, such as fire alarms, burglar alarms, intercoms, etc. The system preferably uses inexpensive, four wire telephone cable and greatly simplifies installation in that only a single cable need be strung through a building. The line is then cut at desired places, and control devices inserted and installed where needed to run to different data devices. If a control device is installed without a corresponding data communication device the system simply sequences over that control device with no appreciable delay or an indentifier is inserted for the data communication device as described hereinafter. Thus, control devices for further use can be installed during the initial installation or during construction, or they may be installed at any time subsequent to construction or installation of the electrical wiring.
The system according to the present invention does not require that a central addressing system send out some sort of code over the data lines requesting each control device to respond. This "dumb" feature allows the use of simple equipment for requesting and transmitting data, and eliminates the need for a "boss" computer to monitor the system's operation. Moreover, since all data devices may listen to the data bus even while awaiting the opportunity to transmit, communication between devices is enabled without intervention by a central monitoring device.
The present construction lends itself to additional types of self-sequencing control devices which are responsive to the cessation of power draw by the device controlled. Included for example are such devices as:
(a) branchers which allow branched cables to be added to the main cable, awaiting cessation of current flow in the branched cable before returning to the main cable and advancing to the next-in-line position;
(b) repeaters which are boosters for amplifying or squaring the data signals on the data bus for long cable runs;
(c) identifiers which are in essence data communication devices incorporated into control means for producing a prescribed identification code on the data bus when it is sequenced but transmitting no data other than a pre-set input. Thus the number sequence can be repeated after passing the identifier which effectively provides a prefix for units in a second section of the system;
(d) terminators which are placed at the end of the circuit to act as a reliability check and indicate by generating a predetermined code such as "00" or the like that the entire sequence of readouts has been completed.
There is thus provided by the present invention an improved method and system for self-sequencing a series of data communication devices to transmit data onto a common data bus. It should be kept in mind that in the preferred embodiment while each data communication device is activated in turn to transmit, each can receive data at any time since they are all connected to the common data bus. Each data communication device is activated in sequence, transmits data onto a data bus for as long as is necessary, deactivates and causes the system to shift to the next station. The sequencing of the system can be initiated by a threshold or trigger voltage, or by such other devices as a computer address or the occurrence of a predetermined event and by monitoring the power drawn by each device, the system is self-sequencing once initiated. The system utilizes a power supply line which both provides power to the data communication device being read and produces the self-sequencing results. Power is only supplied to one data communication device at a time, therefore a small amount of power suffices. Should one control station not yet have a data communication device connected thereto or should the communication device draw no current because of a malfunction, the system automatically skips such station because no current is being drawn. As presented herein, a given control means delivers a supply voltage to its data device and continues to do so as long as the device draws current. It will be clear to those skilled in the art that the control means can equally well deliver a supply current to the device so long as a voltage drop associated with the data device indicates to the control means that data are being communicated and the device is still using the bus.
One of the important features of the invention is that additional data communication devices and/or control stations therefor may be added to the main circuit after it is initially run merely by cutting the cable and inserting the control station and/or encoder at the appropriate place. Further, the electrical cable may be utilized with the same or other power sources at times when the readings are not being carried out, such as fire alarms, burglar alarms, and the like.
Therefore, it is an object of the present invention to provide an improved self-sequencing data communications network.
It is another object of the present invention to provide a system for allocating the use of a common communications or data bus to a series of individual members of a series of data communications devices, wherein each data communication device in the system transmits data onto a common data line or bus and then signals the next data communication device to be activated.
It is yet another object of the present invention to provide a self-sequencing data communications network in which each data communication device in the system is not separately addressed from a common addressing source.
Another object of the present invention is to provide a self-sequencing data communications network in which the cost and complexity of the equipment required to remote locations and unused time is minimized.
Yet another object of the present invention is to provide a self-sequencing data acquisition system in which additional stations can easily be added to the existing system with a minimum of effort and at any point so desired.